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Design your weed

Posted 5/10/2016

Dodder (Cuscuta spp.)

One topic that I often address is — weeds. Those are the things we did not plant, yet often appear to be growing better than those we did! Many of us find them to be one of the biggest difficulties any gardener faces, even gardeners in the desert southwest. It is not only gardeners who must deal with weeds however; farmers, public lands managers, golf course superintendents and virtually anyone who works with plants must face the issues posed by uninvited horticultural guests.

In some of my classes, I ask teams of students to design the “perfect weed.” What could make one plant more of a concern than some other? The answers from the teams were interesting and creative.

For a plant to be a weed, it would first need to be able to survive in the environment. Since we are in the Mojave, we have a unique set of conditions. If a plant required a large amount of water, for example, it would not be a big concern in desert landscapes. Something that was a terrible weed in Louisiana might not be one here.

The perfect weed would need to have some way to reproduce in its environment. This could be by creating vast numbers of seeds, which is one of the ways salt cedar (Tamarix ramosissima) has become such a noxious invader. Seeds of any weed would have to survive until conditions were propitious for germination and growth, which is among the techniques that field bindweed (Convolvulus arvensis) uses to overwhelm a landscape.

Another element that the perfect weed might have would be alternate means of reproduction. Nutsedge is one of the noxious weeds that can wreak havoc on a planted area. This depends less on seeds and more on the tubers that it produces. Anyone who has tried to remove Bermuda grass (Cynodon dactylon) has noticed that it sends out shoots from its jointed stolons, horizontal stems running along the ground.

A perfect weed must be able to obtain crucial nutrients, which are usually limited in a desert environment. Many of the most problematic plants simply outcompete their neighbors, by growing faster or producing larger, more robust root systems. Others sacrifice some or all of their ability to photosynthesize and instead parasitize the plants around them. Dodder (Cuscuta spp.) is an example of a complete parasite. This plant, which some people have described as “silly string,” sends its threads into a plant, and then pulls out essential nutrients from its host.

There are so many ways that a plant can become a perfect weed.

Whether it survives with infrequent water, produces large numbers of seeds or other reproductive propagules, or takes limited nutrients from other plants that cannot compete well, it will be a problem for horticulturists.

No plant is only a weed, but some plants are more likely to become weeds. For any of them, the most important control is to observe the landscape and remove those unwelcome plants before they have a chance to become established and major problems.

Email or call Angela O’Callaghan, Social Horticulture Specialist for University of Nevada Cooperative Extension, at 702-257-5581.